English church and West Frisian tsjerke, but Dutch kerk, Low German Kerk, Kark, and German Kirche

The early Anglo-Frisian and Old Saxon were spoken by intercommunicating populations, which led to shared linguistic traits through assimilation. English and Frisian have a proximal ancestral form in common before their divergence. Geography isolated the settlers of Great Britain from Continental Europe, except from contact with communities capable of open water navigation. This resulted in Old Norse and Norman language influences on Modern English, whereas Modern Frisian was subject to contact with the southernly Germanic populations, restricted to the continent.

It is not thought of as a monolithic proto-language, but rather as a group of closely related dialects that underwent several areal changes in relative unison.[7]

The grouping was first proposed in Nordgermanen und Alemannen (1942) by the German linguist and philologist Friedrich Maurer (1898–1984), as an alternative to the strict tree diagrams which had become popular following the work of the 19th-century linguist August Schleicher and which assumed the existence of an Anglo-Frisian group.[8]

Wolfram Euler (2013), Das Westgermanische [subtitle missing] (West Germanic: from its Emergence in the 3rd up until its Dissolution in the 7th Century CE: Analyses and Reconstruction). 244 p., in German with English summary, Verlag Inspiration Un Ltd., London/Berlin, ISBN978-3-9812110-7-8.